This invention relates to a method of manufacturing cushion materials used for covering the surfaces of seats and other internal members of automobiles and houses, articles of furniture, trunks, clogs and the like, each of such cushion materials being in the form of a sandwich structure including a vinyl coated fabric or like surface material (a first layer), a core or filler pad of conventional foamed polyurethane (a second layer) and a backing material (a third layer), welded together by, for example, high frequency welding.
When, in the production of the cushion material of the three-layer structure by high frequency welding, the melting point of conventional foamed polyurethane used as the filler pad providing the second layer is higher than that of the vinyl coated fabric providing the first layer, the vinyl coated fabric providing the first or surface material layer may be excessively fused or molten during welding of the three layers by application of heat, resulting frequently in such defects that the surface layer bears an undesirable luster or becomes in the form of keloids or is broken off the second layer.
Conventional or usual foamed polyurethane (which will be referred to hereinafter merely as conventional polyurethane) has generally such a property that, when compressed by a pressure applying member while being heated at about 130.degree. C., it starts to deform and is finally fused or molten at about 180.degree. C. Vinyl coated fabric starts to deform at about 110.degree. C. and is finally fused or molten to flow at about 140.degree. C.
In view of the defects pointed out above, low-melting polyurethane or the like has been employed in place of the conventional polyurethane. However, the low-melting polyurethane is twice or more as expensive as the conventional polyurethane, and its physical properties are extremely lower than those of the conventional polyurethane. Therefore, a method has been proposed which comprises introducing a welding material such as powdery nylon to weld areas of conventional polyurethane, and heating the conventional polyurethane at about 110.degree. C. at which the conventional polyurethane would not become molten although the powdery nylon is molten, thereby welding the polyurethane core to the surface material layer by the molten powdery nylon while causing desire deformation of the core of conventional polyurethane. This method has, however, been also defective in view of the material cost of the welding material and the additional step of introduction of the welding material.
For a better understanding of the present invention, this prior art method will be described with reference to FIGS. 1 to 3.
FIG. 1 is a schematic sectional view of a cushion material of three-layer structure including a vinyl coated fabric or like surface material (a first layer) 1, a core or filler pad of conventional polyurethane (a second layer) 2, and a backing material (a third layer) 3. The cushion material includes weld areas 4 welded by pressure applied by a welding member. The prior art method of manufacturing such a cushion material will be described with reference to FIG. 2.
FIG. 2 shows one form of prior art welding equipment used for welding the first, second and third layers. Referring to FIG. 2, a welding member 5 fixedly mounted on a lower plate 9 of a high frequency welding apparatus is formed with a plurality of pressure applying projections 5a the number of which is equal to the number of weld areas 4 of the cushion material. The surface material 1, the conventional polyurethane 2 and the backing material 3 are placed in the above order on the pressure applying projections 5a of the welding member 5, and then, a sheet of insulating paper 6 is placed on the backing material 3. Subsequently, an upper plate 8 of the high frequency welding apparatus having a heating unit 7 fixedly mounted on the lower surface thereof is moved downward to compress the weld areas 4 of the sandwich of the surface material 1, conventional polyurethane 2 and backing material 3 between the heating unit 7 and the pressure appplying projections 5a while heating the weld areas 4 by the heating unit 7, and, at the same time, high frequency heating current is supplied to weld together the three layers at the weld areas 4 shown in FIG. 1.
FIG. 3 is an enlarged detail view of part of the three layers welded together by one of the pressure applying projections 5a of the welding member 5 shown in FIG. 2. In this case, a vinyl coated fabric is used as the surface material 1. It will be seen in FIG. 3 that the portion of the surface material 1 engaged by the corners of the upper end of the pressure applying projection 5a is molten and depends downward to form a deformed surface layer portion 1a which looks like a keloid, or as a result of refoaming or excessive melting, the corresponding portion of the backing of the vinyl coated fabric 1 may be exposed, or an undesirable luster impairing the otherwise beautiful appearance of the entire surface of the surface material 1 may appear on that surface portion of the surface material 1. Also, the portion of the weld area 4 welded by the pressure applying projection 5a may flow out, resulting in a lowered mechanical strength of the weld. Such a cushion material is extremely degraded in quality to such an extent that it is no more marketable.